1. Field of the Invention
The present invention relates generally to a method and apparatus for acquiring a code group in an asynchronous wideband code division multiple access system and, more particularly, to a method and apparatus for acquiring a code group in an asynchronous wideband code division multiple access system, in which code groups are acquired only for the first several slots using receiver diversity with respect to a synchronization channel used for cell search in the asynchronous wideband code division multiple access system, thus reducing code acquisition time and the average number of acquisition operations in an actual multi-path fading area.
2. Description of the Related Art
FIG. 3 is a diagram illustrating the configuration of a synchronization channel of wideband code division multiple access (WCDMA) that was proposed as a standard.
In an asynchronous WCDMA system, respective base stations use different codes. In this system, unlike an asynchronous WCDMA system, although a code used in one base station is shifted, the shifted code does not become a code that is used in some other base station.
In WCDMA, a synchronization channel includes a primary synchronization channel that is used for slot timing synchronization informing a mobile station having no synchronization information of the start point of every slot, and a secondary synchronization channel that is used to acquire frame timing synchronization and determine a primary scrambling code group corresponding to primary scramble codes used by respective base stations.
In WCDMA, one frame of the synchronization channel is 10 msec and is composed of a total of 15 slots. The primary synchronization channel is transmitted during the first 256 chips of each slot (2560 chips). In this case, the same codes are transmitted in respective slots, so that slot timing synchronization can be achieved. The secondary synchronization channel is transmitted during the first 256 chips of each slot (2560 chips), but one of 16 different codes is transmitted in each slot. That is, the received secondary synchronization code values of the secondary synchronization channel can be found through a secondary synchronization channel search process, and the code group of the received codes can be found. The secondary synchronization code values of secondary synchronization channel code groups are illustrated in FIGS. 4A and 4B.
In the secondary synchronization channel, 960 (64*15=960) cases where 64 code groups are L-slot (L=0, 1, 2, 3, 4, . . . , 14)-shifted can be candidates for code group acquisition and frame timing synchronization. A candidate having the highest energy value is determined by comparing a received secondary synchronization channel and a total of 960 candidates, and correlation values with the received secondary synchronization channel are acquired for 15 slot values per candidate. That is, since correlation values corresponding to the length of the 256 chips of one slot value are acquired, correlation operations corresponding to 256 chips*15 slots are necessary for each candidate. Therefore, there occur problems in that a large number of operations, a long search time and a large amount of power consumption are required.
To mitigate the problems, there have been proposed a method of performing calculations on some of the 15 slots of a total of 960 candidates and then performing a sufficient number of operations only on possible candidates for 15 slots, and a method of first acquiring the secondary synchronization code values of the 15 slots of a received secondary synchronization channel and performing pre-processing based on the acquired values, thereby reducing the time that is taken for code group acquisition. However, these methods are disadvantageous in that the expected effect of saving search time cannot be achieved because, in the case where only one antenna is used when calculations are first performed on some of the slots, signals collide against buildings several times in a multi-path fading environment such as in urban centers and, therefore, received signals has been weakened or attenuated. Furthermore, if the receiver diversity for enhancing the reception performance of a terminal by processing signals, which are received through two conventional antennas and a reception Radio Frequency (RF) circuit, in a Digital Signal Processor (DSP) is applied to all of the slots without change, there occur problems in that the number of operations and the amount of current assumption are increased.